Known amplification models for hearing devices have fixed limits for the input level, below which a squelch is applied. In fact, input signals with levels that lie below a predetermined limit will be basically reduced or amplified in a reduced manner, respectively, compared to input signals with levels that lie above a predetermined limit. From this measure, it arises that background noise will not be amplified or only in a reduced manner. As a result thereof, the hearing device user will not be disturbed by excessive amplified background noise, which basically contains no useful information.
A known amplification course is represented in FIG. 1, in which the amplification G is represented as a function of the level S. The amplification G as well as the level S of the input signals are given in the unit decibel (dB). From the course for the amplification G represented in FIG. 1 three ranges I, II and II can be identified, of which the known squelch lies in range I, of which the normal operating range lies in range II and of which an output level limiting is applied in range III. The slope of the course of the amplification G in range II corresponds to a compression needed for the compensation of a hearing loss. In contrast thereto, it is omitted by the course of the amplification G in the range I that surrounding noise will be amplified in correspondence to the particular level, which surrounding noise basically contains no information for the hearing device user. Thereby, it is of importance what value is assigned to a first limit between the range I and II, which limit will be called first knee point SKP1 in the following. In case the value for the first knee point SKP1 is chosen to be too high, the comprehensibility of spoken words will be reduced in very quiet surroundings. In case the value for the first knee point SKP1 is in turn chosen to be too low, interference noise, on the other hand, will not be suppressed enough.
In an analogous manner, the upper limit, which will be identified as second knee point SKP2 in the following, will be fixed between the ranges II and III, the input signals having higher levels as the second knee point SKP2 being limited because one assumes that input signals with higher values than the second knee point SKP2 are felt as painful for the hearing device user, when such input signals are applied, resulting from the amplification applied to the corresponding input level, from which the output level results.
For more progressive algorithms, for which a so-called two-path system with two time constants is applied instead of the above mentioned one-path system, basically the same disadvantage is obtained. The known processes are based on two time constants, namely on a long time constant of typically several seconds and on a short time constant of typically several dozens of milliseconds. While the average amplification is adjusted over the long time constant, the actual syllabic compression is adjusted over the short time constant. Both paths will be added such that in the end the fast path with the short time constant is superimposed over the slow path which determines the actual operating point. The connection is thereby obtained over the operating point determined by the slow path.
A possible course of signal amplification for such a two path system is represented in FIG. 2. On the horizontal and vertical axes, the same values have again been entered in the same unit as in FIG. 1. While the course of the slow path is denoted by Gslow, which sets the actual operating point A1, A2 or A3, the fast path is denoted by Gfast1 or Gfast2 or Gfast3, in which the syllabic compression takes place.
The one-path as well as the two-path system incorporate enormous problems, if the acoustic situation changes. In other words, the known systems can only very badly adapt to changing acoustic situations.
A one-path system can either incorporate a syllabic compression or an automatic volume control (AVC), but not both. The known one-path system has been extensively described by Brian C. J. Moore in his book entitled “Perceptual consequences of cochlear damage” (ISBN 0 19 852330 0, pages 179 to 189).
The two-path system also profits by a reduced amplification below the first knee point SKP1 if the input level is low for a long time. For short speech breaks, the known two-path system cannot reduce the amplification in a sensible manner therefore. If, in a fast path of a two-path system, on the other hand, a further fixed knee point is also introduced with reduced amplification of small input levels, the knee point level for very quiet or very loud signals is adjusted in a wrong manner, i.e. the speech information will not be amplified enough, or noise is amplified too strong. The known two-path systems have been extensively described by Michael Stone et al. in the paper entitled “Comparison of different forms of compression using wearable digital hearing aids” (J. Acoust. Soc. Am. 106(6), December 1999, pages 3603 to 3609).
Therefore, it is an object of the present invention to provide a method for which the disadvantages mentioned above do not occur.